Humans burning fossil fuels have put carbon dioxide into the atmosphere at an unprecedented rate. Carbon dioxide is a greenhouse gas contributing to warming global temperatures and the Intergovernmental Panel on Climate Change (IPCC) predicts that global temperatures will rise by 3 degrees Celsius by 2100. The IPCC assess scientific literature on climate change to create summary reports for policy makers summarizing climate predictions and strategies for mitigation.

What will happen to coral as global warming and ocean acidification progresses? Warmer temperatures impacts corals because corals can only survive in a narrow range of temperatures and when it becomes too warm coral bleaching may occur (figure 1). Corals are further impacted by the amount of carbon dioxide that enters the water. Increasing carbon dioxide, leads to a more acidic ocean. Acidifying impacts corals by decreasing their calcification rate, how readily they grow and build their skeletons.

As a result of these negative repercussions to climate change, scientists are starting to plan for climate change by identifying refugia or areas where corals are predicted to survive in the future. With these areas in mind we can then plan accordingly to conserve regions critical to coral survival. A recent study by Yuan and co-authors tested how tolerant six different coral species are to environments representative of future climate.

Researchers went on cruises in the northern South China Sea in September and October of 2010, 2012 and 2013.The study sites were on the western border of what is called the Coral Triangle that hosts 571 known reef coral species. During each cruise the authors measured important physical parameters, like the temperature and salinity of the water. In addition to the measurements collected during cruises, they incorporated data from othersources in their study to broaden the time frame, such as satellite temperature data spanning from 2003 to 2016. They also used data archives of carbon dioxide air levels in Hong Kong.To be able to see how corals typically found in their field sites might survive in future climate scenarios, they also collected coral fragment samples of six different highly abundant types of corals (figures 2 and 3).

The researchers then brought the coral samples to the lab and allowed to acclimate to their new environment for 2-3 weeks. Researchers subjected different types of coral to varying acidity and levels of carbon dioxide in the air. As a measure of how well corals responded to different environments, they compared the weight of the coral before changing the environmental conditions to the weight of the coral after 30 days. Ultimately the goal of the study was to combine the results from the laboratory experiments with the field measurements to see how corals might survive in these sites considering current environmental change combined with future predictions.

Figure 3. Image of one of the types of massive corals used in this study called Favorites complanata. Image Credit: https://en.wikipedia.org/wiki/Favites_complanata.

Results

In the Field

Since this study has two main components, let’s first discuss the results from each of their study sites. All three locations showed an increase in temperature, more rapid than what is predicted for global average by the IPCC. The authors also observed an increase in acidity each year. The amount of carbon dioxide also increased each year in Hong Kong waters and only this increase in carbon dioxide resulted in 20% of the change in acidity. The increase in acidity was also more rapid in Hong Kong waters than predicted by the IPCC. This may decrease coral calcification in these waters in the future, they suggested at the end of this century. Since their study sites spanned both tropical and subtropical waters, they identified subtropical waters as a potential coral refugia. Subtropical waters represent a potential refuge because these waters were 3 degrees Celsius cooler than tropical waters.

In the Lab

Two types of coral grew at a much slower rate due to the high levels of carbon dioxide they were subjected to in the lab: massive coral species and branching coral. Temperature also had negative effects. The researchers suggest that whether or not some coral species can survive increasing temperatures depends on how quickly the species will adapt.

Summary/Big Picture

Overall, corals in the South China Sea are likely to face challenging conditions in the future and we need to take measures to protect them. Additionally, the authors suggest that there are many other factors that they did not consider that should also be taken into account – pollution, exploitation, and habitat loss.

I am a first year graduate student at the Graduate School of Oceanography at University of Rhode Island. I use robots and satellites to research the biological carbon pump, which is a series of processes that transfer carbon dioxide from the atmosphere into the deep ocean where the carbon is stored for long periods of time. I am particularly interested in the use of oxygen measurements to better understand how much carbon-containing material is produced by phytoplankton, tiny marine organisms, and is available for transport to the deep ocean. Learning about how much carbon the ocean stores through these processes is important to improve predictions about how climate change will impact the ocean.